net: skbuff: propagate shared-frag marker through frag-transfer helpers

cve CVE-2026-46300
commit-author Hyunwoo Kim <imv4bel@gmail.com>
commit -
commit-source https://lore.kernel.org/all/agW4vC0r8QOUKtRT@v4bel/
upstream-diff |
  The upstream v3 patch does not apply cleanly to 4.18 due to three
  differences:
  1. net/core/gro.c does not exist on 4.18. Both skb_gro_receive()
     and skb_gro_receive_list() are in net/core/skbuff.c.
  2. skb_shift() context differs — 4.18 uses manual skb->len/data_len
     adjustments, upstream uses skb_len_add().
  3. 4.18 uses the older tx_flags/SKBTX_SHARED_FRAG API instead of
     the newer flags/SKBFL_SHARED_FRAG API introduced in later kernels.
  The actual code changes are identical to upstream, adapted for the
  4.18 API.

Three frag-transfer helpers (__pskb_copy_fclone(), skb_try_coalesce(),
and skb_shift()) fail to propagate the SKBFL_SHARED_FRAG bit in
skb_shinfo()->flags when moving frags from source to destination.
__pskb_copy_fclone() defers the rest of the shinfo metadata to
skb_copy_header() after copying frag descriptors, but that helper
only carries over gso_{size,segs,type} and never touches
skb_shinfo()->flags; skb_try_coalesce() and skb_shift() move frag
descriptors directly and leave flags untouched.  As a result, the
destination skb keeps a reference to the same externally-owned or
page-cache-backed pages while reporting skb_has_shared_frag() as
false.

The mismatch is harmful in any in-place writer that uses
skb_has_shared_frag() to decide whether shared pages must be detoured
through skb_cow_data().  ESP input is one such writer (esp4.c,
esp6.c), and a single nft 'dup to <local>' rule -- or any other
nf_dup_ipv4() / xt_TEE caller -- is enough to land a pskb_copy()'d
skb in esp_input() with the marker stripped, letting an unprivileged
user write into the page cache of a root-owned read-only file via
authencesn-ESN stray writes.

Set SKBFL_SHARED_FRAG on the destination whenever frag descriptors
were actually moved from the source.  skb_copy() and skb_copy_expand()
share skb_copy_header() too but linearize all paged data into freshly
allocated head storage and emerge with nr_frags == 0, so
skb_has_shared_frag() returns false on its own; they need no change.

The same omission exists in skb_gro_receive() and skb_gro_receive_list().
The former moves the incoming skb's frag descriptors into the
accumulator's last sub-skb via two paths (a direct frag-move loop and
the head_frag + memcpy path); the latter chains the incoming skb whole
onto p's frag_list.  Downstream skb_segment() reads only
skb_shinfo(p)->flags, and skb_segment_list() reuses each sub-skb's
shinfo as the nskb -- both p and lp must carry the marker.

Fixes: cef401d ("net: fix possible wrong checksum generation")
Fixes: f4c50a4 ("xfrm: esp: avoid in-place decrypt on shared skb frags")
Reported-by: William Bowling <vakzz@zellic.io>
Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>

Author: shreeya-patel98

net/core/skbuff.c

@@ -1807,6 +1807,7 @@ struct sk_buff *__pskb_copy_fclone(struct sk_buff *skb, int headroom,
 			skb_frag_ref(skb, i);
 		}
 		skb_shinfo(n)->nr_frags = i;
+		skb_shinfo(n)->tx_flags |= skb_shinfo(skb)->tx_flags & SKBTX_SHARED_FRAG;
 	}
 
 	if (skb_has_frag_list(skb)) {
@@ -3696,6 +3697,8 @@ int skb_shift(struct sk_buff *tgt, struct sk_buff *skb, int shiftlen)
 	tgt->ip_summed = CHECKSUM_PARTIAL;
 	skb->ip_summed = CHECKSUM_PARTIAL;
 
+	skb_shinfo(tgt)->tx_flags |= skb_shinfo(skb)->tx_flags & SKBTX_SHARED_FRAG;
+
 	/* Yak, is it really working this way? Some helper please? */
 	skb->len -= shiftlen;
 	skb->data_len -= shiftlen;
@@ -4119,6 +4122,8 @@ int skb_gro_receive_list(struct sk_buff *p, struct sk_buff *skb)
 	p->truesize += skb->truesize;
 	p->len += skb->len;
 
+	skb_shinfo(p)->tx_flags |= skb_shinfo(skb)->tx_flags & SKBTX_SHARED_FRAG;
+
 	NAPI_GRO_CB(skb)->same_flow = 1;
 
 	return 0;
@@ -4586,10 +4591,12 @@ int skb_gro_receive(struct sk_buff *p, struct sk_buff *skb)
 	p->data_len += len;
 	p->truesize += delta_truesize;
 	p->len += len;
+	skb_shinfo(p)->tx_flags |= skbinfo->tx_flags & SKBTX_SHARED_FRAG;
 	if (lp != p) {
 		lp->data_len += len;
 		lp->truesize += delta_truesize;
 		lp->len += len;
+		skb_shinfo(lp)->tx_flags |= skbinfo->tx_flags & SKBTX_SHARED_FRAG;
 	}
 	NAPI_GRO_CB(skb)->same_flow = 1;
 	return 0;
@@ -5576,6 +5583,8 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
 	       from_shinfo->frags,
 	       from_shinfo->nr_frags * sizeof(skb_frag_t));
 	to_shinfo->nr_frags += from_shinfo->nr_frags;
+	if (from_shinfo->nr_frags)
+		to_shinfo->tx_flags |= from_shinfo->tx_flags & SKBTX_SHARED_FRAG;
 
 	if (!skb_cloned(from))
 		from_shinfo->nr_frags = 0;